Construction of a Large-Diameter Pipeline in Soft Soils between Two Reservoirs Using Alternative Project Delivery Method

The City of Phoenix used an alternate project delivery method to complete the Reservoir Interconnect Pipeline at the Deer Valley Water Treatment Plant (WTP). This pipeline provides the City with the ability to operate the two existing 20 million gallon (MG) finished water reservoirs at the WTP. This improved flexibility eliminated short circuiting within either of the reservoirs and allows them to be operated in parallel or independently. The project needed to be completed within a short time frame under challenging site conditions and constraints. The City chose construction manager at risk (CMAR) project delivery method with Wilson Engineers designing the project and Kiewit Infrastructure constructing the project as the CMAR contractor. Many alternative construction materials and methods were evaluated to determine the most cost-efficient, safe, and quality-driven solutions to the project challenges. The interconnect pipeline installation was constrained by a compressed schedule and challenging site conditions. The 96-in. reinforced concrete cylinder pipe (RCCP), manufactured in accordance with the AWWA C300 standard, was installed in soft soil conditions at a depth ranging from 20 to 32 ft between two operating reservoirs 50 ft apart. Two junction structures were constructed to connect the new pipeline to the existing pipelines during the annual plant inspection period. Kiewit collaborated with the City of Phoenix staff and consulting engineer to develop, design, and construct a safe and effective shoring. The chosen system allowed the flexibility to overexcavate unsuitable soils and improve the subgrade to support the pipeline, while allowing unimpeded access to install the pipe. Soils between the reservoirs were continuously monitored during excavation, pipe installation, and backfill operations to ensure the stability of the existing structures. The 96-in. pipe was installed using a mobile trolley type gantry crane system. The trolley was fabricated on site and supported by two concrete foundations placed along the length of the excavation. The pipe sections were swung into the excavation with a crane, picked up by the trolley, and pushed down the trench to its final installed location. Extensive planning, prior to and during the course of construction, resulted in the project being completed and the WTP back in operation ahead of schedule and under budget.